Microwave-Fluidic Continuous Manufacturing of Ultrasmall Silver Nanoparticles in a Polycaprolactone Matrix as Antibacterial Coatings

Abstract

Fast, energy-efficient, and continuous manufacturing of nanoparticles (NPs) with controlled size and distribution in polymer matrices is challenging. Herein, a microwave-powered dual-injection continuous flow reactor is presented to prepare silver NP (AgNP)/polycaprolactone (PCL) nanocomposites (AgNP/PCL NCs). Ultrasmall spherical AgNPs (US-AgNPs, 1.86 ± 0.77 nm) can be manufactured in the PCL matrix in less than 3 min at ∼35 °C by applying 60 W microwave power and a combined flow rate of 1.25:1.25 mL/min (Pump1:Pump2). The effect of NP size and amount on the thermal, optical, and antimicrobial properties and the crystallinity of NCs are discussed. The NC crystallinity is independent of the NP's size and amount, while the NC film roughness is highly dependent on NP size. The antibacterial activity of the US-AgNPs-containing NC toward Escherichia coli (∼98.2%), Pseudomonas aeruginosa (∼98.2%), and Staphylococcus aureus (∼99.1%) is higher than big AgNP-containing NCs (82.3%, 85.7%, and 92.3%, respectively), signifying a strong NP size dependency instead of Ag concentration.

Figure 1

Illustration of a double-pump continuous MWFS to prepare NCs.

Figure 2

Effect of MW power. a) Photographs of as-prepared NC1–6 solutions, b) UV–vis spectra, and c) particle size analysis (by DLS) of NC1–6.

Figure 3

Effect of flow rate. a) Photographs of as-prepared NC7–11 solutions, b) UV–vis spectra, and c) particle size analysis (by DLS) of NC7–11.

Figure 4

Effect of AgNO₃ and PCL concentrations. a) Photographs of as-prepared NC12–16 solutions, b) UV–vis spectra, and c) particle size analysis (by DLS) of NC12–16.

Figure 5

TEM images of NC8 (a, b) and NC11 (d, e) at different magnifications and the mean particle size distribution of AgNPs in NC8 (c) and NC11 (f).

Figure 6

a) FTIR, b) TGA-DTA, and c) XRD results of PCL, NC8, and NC11. Inset in b shows the DTA vs temperature plot. Insets in c display the enlarged regions of corresponding PCL (black), NC8 (red), and NC11 (NC11). The labels correspond to the observed peak positions and diffraction planes.

Figure 7

Colony-forming units (CFU) of E. coli, S. aureus, and P. aeruginosa after exposure to uncoated glass and PCL, NC8, and NC11 films.